Unit two. The Living Cell
6.5. Building New Molecules
The Calvin Cycle
Stated very simply, photosynthesis is a way of making organic molecules from carbon dioxide (CO2). To build organic molecules, cells use raw materials provided by the light-dependent reactions:
1. Energy. ATP (provided by the ETS of photosystem II) drives the endergonic reactions.
2. Reducing power. NADPH (provided by the ETS of photosystem I) provides a source of hydrogens and the energetic electrons needed to bind them to carbon atoms. A molecule that accepts an electron is said to be reduced, as will be discussed in detail in chapter 7.
The actual assembly of new molecules employs a complex battery of enzymes in what is called the Calvin cycle, or C3 photosynthesis (C3 because the first molecule produced in the process is a three-carbon molecule). The Calvin cycle takes place in the stroma of the chloroplasts. The NADPH and the ATP that were generated by the light-dependent reactions are used in the Calvin cycle to build carbohydrate molecules. In the Key Biological Process illustration below, the number of carbon atoms at each stage is indicated by the number of balls. It takes six turns of the cycle to make one six-carbon molecule of glucose. The process takes place in three stages, highlighted in the three panels of the Key Biological Process illustration below. These three stages are also indicated by different-colored pie-shaped pieces in the more detailed look at the Calvin cycle provided in figure 6.10. Both figures indicate that three turns of the cycle are needed to produce one molecule of glyceraldehyde 3-phosphate. In any one turn of the cycle, a carbon atom from a carbon dioxide molecule is first added to a five-carbon sugar, producing two three- carbon sugars. This process, highlighted by the dark blue arrow in panel 1 of the Key Biological Process illustration and the blue pie-shaped area in figure 6.10, is called carbon fixation because it attaches a carbon atom that was in a gas to an organic molecule.
Figure 6.10. Reactions of the Calvin cycle.
For every three molecules of CO2 that enter the cycle, one molecule of the three-carbon compound glyceraldehyde 3-phosphate (G3P) is produced. Notice that the process requires energy stored in ATP and NADPH, which are generated by the light-dependent reactions. This process occurs in the stroma of the chloroplast. The large 16-subunit enzyme that catalyzes the reaction, RuBP carboxylase, or rubisco, is the most abundant protein in chloroplasts and is thought to be the most abundant protein on earth.
Then, in a long series of reactions, the carbons are shuffled about. Eventually some of the resulting molecules are channeled off to make sugars (shown by the dark blue arrows in panel 2 of the Key Biological Process illustration and at the bottom of the cycle within the purple colored area in figure 6.10). Other molecules are used to re-form the original five-carbon sugar (the dark blue arrow in panel 3 of the Key Biological Process illustration and the light-red-colored area in figure 6.10), which is then available to restart the cycle. The cycle has to “turn” six times in order to form a new glucose molecule, because each turn of the cycle adds only one carbon atom from CO2, and glucose is a six-carbon sugar.
Recycling ADP and NADP+
The products of the light-dependent reactions, ATP and NADPH, feed into the light-independent reactions of the Calvin cycle to make sugar molecules. To keep photosynthesis moving along, the cells must continually supply the light-dependent reactions with more ADP and NADP+. This is accomplished by recycling these products from the Calvin cycle. After the phosphate bonds are broken in ATP, ADP is available for chemiosmosis. After the hydrogens and electrons are stripped from NADPH, NADP+ is available to cycle back to the electron transport system of photosystem I.
Key Learning Outcome 6.5. In a series of reactions that do not directly require light, cells use ATP and NADPH provided by photosystems II and I to assemble new organic molecules.